Reduction projection exposure using ultraviolet light has conventionally been performed as a printing (lithography) method of manufacturing a fine semiconductor element, such as a semiconductor memory or logic circuit.
The minimum size capable of transfer by reduction projection exposure is proportional to the wavelength of light used for the transfer and inversely proportional to the numerical aperture of the projection optical system. To transfer a fine circuit pattern, the wavelength of exposure light for use is being shortened. The wavelength of ultraviolet rays for use is becoming shorter, to a mercury-vapor lamp i-line (wavelength: 365 nm), a KrF excimer laser beam (wavelength: 248 nm), and an ArF excimer laser beam (wavelength: 193 nm).
However, as semiconductor elements rapidly shrink in feature size, there is a limit to lithography using the above-described ultraviolet light. To efficiently print a very fine circuit pattern, smaller than 0.1 μm, a reduction projection exposure apparatus using extreme ultraviolet light (EUV light) whose wavelength is much shorter than the above-described ultraviolet light, i.e., as short as about 10 to 15 nm, is being developed.
Light in the EUV wavelength region is greatly absorbed by a substance. The use of an optical lens system, which utilizes light refraction and matches a visible light region, or the wavelength region of the above-described ultraviolet light, is not practical, and a reflecting optical system is adopted. A reticle is a reflective reticle on which a pattern to be transferred onto a mirror is formed by an absorber.
Reflecting optical elements, which form the exposure apparatus using EUV light include a multilayer mirror and an oblique incidence total reflection mirror. The real part of the refractive index of a mirror material with respect to light in the EUV wavelength region is slightly smaller than one. Therefore, to allow the mirror surface to totally reflect EUV light, it must be applied to the reflecting surface of the mirror at a small incidence angle. In general, oblique incidence at several degrees from the reflecting surface provides a high reflectance of several tens of percent or more. However, since the degree of freedom in optical design is low, it is difficult to use the total reflection mirror in the projection optical system.
An EUV light mirror, which receives EUV light at an incidence angle almost close to normal incidence is a multilayer mirror obtained by alternately stacking, on the reflecting surface, two types of substances having different optical constants. Molybdenum and silicon are alternately stacked on the surface of a glass substrate polished into a precisely planarized state. The layer thickness is, e.g., 0.2 nm for the molybdenum layer and about 0.5 nm for the silicon layer. The number of stacked layers is about twenty pairs. A value as the sum of the thicknesses of layers of the two types of substances will be called a film cycle. In this example, the film cycle is 0.2 nm+0.5 nm=0.7 nm.
When EUV light is applied to the multilayer mirror, EUV light having a specific wavelength is reflected.
Letting θ be the incidence angle, λ be the wavelength of EUV light, and d be the film cycle, only EUV light with a narrow bandwidth centered on λ, which approximately satisfies Bragg's equation:2×d×sin θ=λis efficiently reflected. At this time, the bandwidth is about 0.6 to 1 nm.
The reflectance of a multilayer mirror to EUV light is about 0.7 at a maximum, so the light amount loss at each multilayer mirror is very large. EUV light, which is not reflected by the multilayer mirror, is absorbed in the multilayer film or the substrate of the multilayer mirror, and most of the energy is converted into heat. Moreover, the multilayer mirror may expand or deform due to the influence of the heat, resulting in a deterioration in performance of the entire optical system.
To expose a wide exposure region with a minimum number of mirrors, it is proposed to simultaneously scan a reticle (original) and wafer (exposure target) to project and to form, by exposure, a reticle pattern onto the wafer. FIG. 9 shows the structure of a scanning exposure apparatus (a so-called scanner) to cope with this technique. The scanning exposure apparatus is constituted by an EUV light source, an illumination optical system, a reflective reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system, and a vacuum system.
As the EUV light source, e.g., a laser plasma source is used. A target material provided by a target providing unit installed outside a vacuum vessel is irradiated with a high-intensity pulse laser beam to generate a high-temperature plasma, and EUV light, which is emitted by the plasma and has a wavelength of, e.g., about 13 nm, is utilized. The target material is a metal thin film, inert gas, droplets, or the like, and is supplied into the vacuum vessel by a means such as a gas jet. To increase the average intensity of emitted EUV light, the repetition frequency of the pulse laser is preferably high. The pulse laser is generally operated at a repetition frequency of several kHz.
The illumination optical system comprises a plurality of multilayer mirrors or oblique incidence mirrors, and an optical integrator. A collection mirror on the first stage collects EUV light almost isotropically emitted from a laser plasma. The optical integrator uniformly illuminates a reticle at a predetermined numerical aperture. An aperture for limiting a region illuminated on the reticle plane to an arcuate shape is formed at a position conjugate to the reticle of the illumination optical system.
The projection optical system uses a plurality of mirrors. A smaller number of mirrors provides a higher EUV light utilization efficiency, but makes aberration correction difficult. The number of mirrors necessary for aberration correction is about four to six. The reflecting surface of the mirror has a convex or concave spherical or aspherical shape. The numerical aperture NA is about 0.1 to 0.2.
The mirror is fabricated by grinding and polishing a substrate made of a material with a high rigidity, a high hardness, and a low thermal expansion coefficient, such as low-expansion-coefficient glass or silicon carbide, into a predetermined reflecting surface shape, and forming multilayer films of molybdenum and silicon on the reflecting surface. If the incidence angle is not constant, depending on the position within the mirror plane, the reflectance of a multilayer film with a predetermined number of film cycles increases depending on the position, shifting the wavelength of EUV light, as is apparent from Bragg's equation. To prevent this, the film cycle distribution must be set such that EUV light having the same wavelength is efficiently reflected within the mirror plane.
The reticle and wafer stages have mechanisms for scanning these stages in synchronism with each other, at a velocity ratio proportional to the reduction magnification. Let X be the scanning direction within the reticle or wafer plane, Y be the direction perpendicular to the scanning direction, and Z be the direction perpendicular to the reticle or wafer plane.
A reticle is held by a reticle chuck on the reticle stage. The reticle stage has a driving mechanism of moving the reticle stage along the X direction at a high speed. The reticle stage also has fine moving mechanisms in the X direction, Y direction, Z direction, and rotational directions around these axes, and can align a reticle. The position and posture of the reticle stage are measured by a laser interferometer, and controlled on the basis of the measurement results.
A wafer is held on the wafer stage by a wafer chuck. The wafer stage has a mechanism of moving the wafer stage along the X direction at a high speed, similar to the reticle stage. The wafer stage also has fine moving mechanisms in the X direction, Y direction, Z direction, and rotational directions around these axes, and can align a wafer. The position and posture of the wafer stage are measured by a laser interferometer, and controlled on the basis of the measurement results.
An alignment detection optical system measures the positional relationship between the reticle position and the optical axis of the projection optical system, and the positional relationship between the wafer position and the optical axis of the projection optical system. The positions and angles of the reticle and wafer stages are set such that a reticle projection image coincides with a predetermined position on a wafer.
The focus position in the Z direction within the wafer plane is measured by a focus position detection optical system, and the position and angle of the wafer stage are controlled. The wafer plane always keeps a position at which the projection optical system images during exposure.
At the end of one scanning exposure on a wafer, the wafer stage moves step by step in the X and Y directions to the next scanning exposure start position. The reticle and wafer stages are sync-scanned again in the X direction at a velocity ratio proportional to the reduction magnification of the projection optical system.
In this way, a sync scanning operation is repeated (step and scan), while the reduction projection image of a reticle is formed on a wafer. As a result, the reticle transfer pattern is transferred onto the entire wafer surface. EUV light is strongly absorbed by a gas. For example, when EUV light having a wavelength of 13 nm propagates 1 m through a space filled with 10-Pa air, the transmittance of the EUV light is about 50%. Similarly, the transmittances of the EUV light upon its propagation by 1 m through spaces filled with 10-Pa gases, e.g., helium, argon, and hydrogen, are about 88%, 71%, and 98%, respectively. In order to avoid EUV light absorption by a gas, the space must be purged with helium with a high transmittance, most of the space where EUV light propagates must be set at a pressure of 10−1 Pa or less, and preferably, 10−3 Pa or less, and the partial pressure of gases (e.g., oxygen and water) having low transmittances must be minimized.
When molecules including carbon, such as hydrocarbons, are left in a space where an optical element irradiated with EUV light is arranged, carbon gradually attaches to the surface of the optical element due to light irradiation. The attached carbon absorbs the EUV light to undesirably decrease the reflectance. In order to prevent carbon from attaching to the optical element, the partial pressure of molecules, including carbon in the space where the optical element irradiated with EUV light is arranged, must be kept at 10−4 Pa or less, and preferably, 10−6 Pa or less.
An exposure apparatus repeats the following operations. That is, the exposure apparatus loads, from outside, a semiconductor wafer coated with a resist serving as a photosensitive agent. The exposure apparatus scans the semiconductor wafer and a reticle, and transfers the pattern of the reticle onto the semiconductor wafer. The exposure apparatus then unloads the exposed semiconductor wafer. The exposure apparatus includes a large number of driving mechanisms and may cause outgassing by friction to result in a decrease in transmittance of a mirror.
A wafer stage includes driving mechanisms, such as a scanning exposure moving mechanism and a wafer transport mechanism, and has a large surface area. Outgassing cannot be eliminated from a component having such a large surface area. Therefore, it is difficult to set an exposure space in a high vacuum.
The resist applied to the wafer is an organic substance, although it is heated and baked before exposure. When the resist is arranged in a vacuum, an organic substance, which forms the resist, or hydrocarbons, as the decomposed substance, are produced and diffused in the apparatus set in the vacuum. A wafer is loaded from the outer atmosphere to the exposure apparatus. During loading of the wafer, it is difficult to remove, within a short period of time, the air component containing moisture attaching to the wafer. The moisture is gradually desorbed and diffused in the vacuum. It is, therefore, very difficult to maintain the high vacuum due to outgassing from the wafer and the resist.
It is possible to set a high vacuum using a large-capacity vacuum pump, but components in the vacuum space become an issue. That is, molecules including carbon and moisture should not be diffused, particularly in the space in which the mirror and reticle are arranged.
As a countermeasure for protecting the mirror from outgassing from the wafer, or the like, a space is formed to surround the mirror, arranged between the reticle and the wafer, in Japanese Patent Application Laid-Open No. 2000-058433. More specifically, a gas (hydrogen, argon, or krypton), which does not substantially absorb EUV light is blown, for the wafer, toward the side surface of a conical opening formed at the exit of light from the space to the wafer.
Along with operation of the wafer stage, the conductance between the conical opening and the wafer stage changes, to change the amount of inert gas blown to the stage space and the projection optical system space. This leads to instability in the pressure of each space and the exposure amount on the wafer surface. In addition, the inert gas flowing toward the wafer surface makes contaminants diffuse in a chamber which accommodates the wafer stage. This contaminates the sensor surface arranged on the wafer stage.